New York: In an observation, a study finds that mass killings and school shootings in the US are contagious - meaning one incident inspires other similar incidents.

"The hallmark of contagion is observing patterns of many events that are bunched in time, rather than occurring randomly in time," said author Sherry Towers from Arizona State University.

Scientists from Arizona State and Illinois University examined databases on past mass killings and school shootings in the US and fit a contagion model to determine if these tragedies inspired similar events in the near future.

They found that mass killings - events with four or more deaths -- and school shootings create a period of contagion that lasts an average of 13 days.

Roughly 20 to 30 percent of such tragedies appear to arise from contagion.

The analysis was inspired by actual events in Towers` life. "In January 2014 I was due to have a meeting with a group of researchers at Purdue University," Towers said.

"That morning there was a tragic campus shooting and stabbing incident that left one student dead. I realised there had been three other school shootings in the news in the week prior."

"I wondered if it was just a statistical fluke, or if somehow through news media those events were sometimes planting unconscious ideation in vulnerable people for a short time after each event."

The researchers found that previous studies have shown that suicide in youth can be contagious, where one suicide in a school appears to spark the idea in other vulnerable youths to do the same.

"Mass killings and school shootings that attract attention in the national news media can potentially do the same thing, but at a larger scale," Towers said.

"While we can never determine which particular shootings were inspired by unconscious ideation, this analysis helps us understand aspects of the complex dynamics that can underlie these events."

On average, mass killings involving firearms occur approximately every two weeks in the US, and school shootings occur on average monthly.

The team found that the incidence of these tragedies is significantly higher in states with a high prevalence of firearm ownership.